1. Field of the Invention
The present invention relates to electronic cards, and more specifically to electronic card identification methods for use with electronic cards that communicate with an interrogation unit.
2. Description of Related Art
Typically, electronic cards respond to interrogation messages from an interrogation unit. Such a process is generally carried out in a system that includes a fixed interrogation unit which can dialogue (i.e., exchange messages) with electronic cards which are mobile with respect to the interrogation unit. In other systems, the interrogation unit is mobile and the electronic cards are fixed. In still other systems, both the interrogation unit and the electronic cards are mobile. In the present description, an electronic card includes any type of "card" such as an electronic module, a badge, or a chip card that can be carried on the person, in a vehicle, on an apparatus, on livestock, or the like. Further, the interrogation unit can be contained in any type of terminal such as a base station, an access control terminal, a stopless toll terminal, or the like.
The dialogue between the interrogation unit and the electronic cards is a contactless type of dialogue. For example, the messages are sent via a radio-frequency or microwave electromagnetic carrier wave. The transmission channel is thus formed by the ambient surroundings. It must therefore be time shared in accordance with a communication protocol between the unit and the cards, in particular to avoid collisions between messages sent to or coming from different electronic cards.
The protocol generally used is of the master/slave type. Indeed, each exchange of message between the interrogation unit (master) and a given card (slave) is initiated by the interrogation unit. The cards are maintained in a specific state, referred to as the standby state, until awakened (activated) by a wake-up message (call) sent by the unit. The latter then sends a command message. When the command is executed, the unit sends a standby message to the card, causing the latter to return to the standby state. In this way, in normal operation, at most only one card is activated at a time to receive a command message. Any other cards that are present are not affected by this command message.
An identification number is allocated univocally to each electronic card. In other words, a unique identification number is associated with each electronic card. This allocation can be made statically or dynamically. In the static case, an identification number is permanently allocated to each card, for example at the time of its manufacture. In the dynamic case, an identification number is randomly generated and allocated to each card when it comes within range of a zone for exchanging messages. Because of the mobility of the electronic cards and/or because of its own mobility, the interrogation unit has no immediate way of knowing whether electronic cards are within the zone for exchanging messages.
In particular, the interrogation unit does not know how many cards are present and above all, it does not know which of the cards are present. It has been proposed to enable the interrogation unit to identify the electronic cards that are present in the zone for exchanging messages. In the following description, this zone shall be called the investigation zone, given that it is a zone within which the electronic cards present are identified. The identification of an electronic card present requires a determination of its identification number.
The conventional processes all operate under the same principle. An interrogation message is sent by the interrogation unit to all the cards present in the investigation zone. All the cards present, or just those which are authorized to respond, then send a response message. The interpretation of the response messages received makes it possible to determine the identification number of all of the cards present, either directly or following an iterative procedure. This exchange of messages necessary for identifying the cards present is referred to as the identification procedure.
The conventional processes differ from one another first by the content of the response message and also by the sequence of response messages sent by the different cards. In order to avoid the problem of possible collisions between the response messages sent by two of more of the cards present, processes have been proposed in which the cards respond to an interrogation message by sending their identification number in a specific time slot within a sequence of n time slots (where n is a whole number greater than or equal to two). Each card is allocated a unique specific time slot, possibly dynamically (i.e., as a function of the number of cards effectively present), so as to be the only card to send a response message inside that time slot.
Another conventional identification process involves reconstituting the identification numbers of the cards present in blocks of Q bits following a branch-structured iterative algorithm in which the response message sent by the cards present (and authorized to respond) includes at least one service bit sent inside a narrow time slot whose positioning within a sequence of 2.sup.N successive and identical time slots indicates the value of a block of bits not yet identified by its identification number.
Such a process is described in French patent application No. 98-01309, which is assigned to STMicroelectronics S.A. of France. While this process provides good results, improved electronic card identification methods are still desired.
A characteristic common to the above-described processes is that the electronic cards respond by sending a response message, whatever its nature, in a given time slot within a sequence of time slots. In view of the synchronization constraints between the interrogation unit and the responding cards, the width of a time window is in practice about a hundred times greater than the time taken to send a response message. The process described in the above-mentioned French patent application improves the process insofar as it allows the use of narrower time slots. Nevertheless, the time slots sequences overall still occupy a relatively long time period. During this period, the interrogation unit waits for messages and cannot dialogue with any card, even one that has already been the identified, since the channel is "reserved" for the transmission of response messages from the card.